Bubble level meter and related method

ABSTRACT

The invention concerns bubble level meter having a pneumatic tube connected to a gas generator and to a pressure sensor. A deflecting valve is interposed along the pneumatic tube, for deflecting the air outlet at a predetermined height from the usual lower outlet of the tube. A control circuit controls the deflecting valve and the gas generator based on predetermined set values, processes the pressure measurements obtained from the pressure sensor, checks the calibration of the sensor on the basis of the measurements and the differences in height between the tube outlet, and generates level data based on the processed pressure measurements and a calibrating coefficient. Another valve interposed between the sensor and the tube and monitored by the circuit control, enables to take into account the possible drift of the sensor.

FIELD OF THE INVENTION

[0001] The present invention relates to a bubble level meter havingimproved stability and measurement accuracy, and a method for adapting abubble level meter to improve pressure measurements. Such a level meteris particularly useful for monitoring the water level of lakes and forany other application requiring the measurement of a liquid level.

BACKGROUND

[0002] Bubble level meters are used since many years in the field ofhydrostatic pressure measurements. They are used as a result of theirsimplicity, their efficiency, their long-term reliability and theirgeneral accuracy in many fields such as industrial, geotechnical, oil,marine, hydrographical reservoir management, etc.

[0003] The basic principle of a bubble level meter mainly consists inopposing the pressure exerted by a water column by means of an externalpressure source, generally air, until a balanced pressure or an equalpressure between the water column and the external pressure source isobtained. The external pressure source then becomes the measurementreference which, after conversion of the measured pressure, provides aheight or level measurement. The conversion depends on the density ofthe liquid to be measured. The pressure is measured using sensors ofmany sorts, such as electric, electronic, optical, pneumatic,mechanical, most of which using a more or less rigid diaphragm subjectedto the pressure to be measured. The pressure sensors are usuallyinitially calibrated in laboratory by their manufacturer. This is howthe calibration coefficients and factors used to calculate the pressureapplied on the diaphragm are determined.

[0004] Among all the characteristics that the manufacturer of thepressure sensor will determine, the sensitivity coefficient and theoffset factor of the pressure sensor are the most important ones toobtain an accurate reading of the pressure applied on the diaphragm.Unfortunately, with the passing of time, or for reasons of design, thiscalibration sensitivity coefficient may vary during the lifetime of thepressure sensor, typically unbeknown to the user. Certain factors mayinfluence the measurements, such as the atmospheric pressure, an unequaldensity in the water column, temperature variations, humidity,corrosion, vibrations, etc. Furthermore, mechanical components or theelectrical or electronic interfaces connected to the pressure sensor mayhighly affect the sensitivity coefficient. It is difficult to controlall these components. This error phenomenon on the sensitivitycoefficient can be verified, provided that the pressure sensor issubjected to a new calibration in laboratory or on site, with equipmentwhich is very specialized at the present time. In any cases, this isvery impractical. Since it is highly difficult to know when thesensitivity coefficient has changed due to unpredictable phenomenaoccurring in time, it is thus possible that the measurements taken bythe apparatus be erroneous, which may involve very seriousrepercussions.

[0005] Another important point is the offset factor of the pressuresensor, which causes an error on the final result of the level reading.Contrary to the sensitivity coefficient which modifies the calibrationslope of the sensor, the offset introduces a residual value whichprevents the sensor from having an initial zero value for a pressuremeasured at zero point. The large majority of pressure sensors have aninitial offset factor during their manufacture, which must be consideredduring the calculations based on the pressure measurements. Moreover,the offset factor generally changes with the operation time of thesensor. In the same way as the error due to a change of the sensitivitycoefficient, that due to the offset factor is also important andsignificant on the final result. To determine the offset factor of thesensor during its operation time, it is very important to have the sameinitial pressure conditions.

[0006] The level meters are often installed in remote locations whichare difficult to access. The doubt on a level measurement reading or,worse, an erroneous reading, may cause irreparable damages. The costsassociated with the transportation of personnel for checking the levelmeter are often huge and represent an amount higher than the price of anew apparatus.

[0007] Known in the art are U.S. Pat. No.: 3,729,997 (Luke); U.S. Pat.No. 3,751,185 (Gottliebson et al.); U.S. Pat. No. 3,987,675 (Harrison);U.S. Pat. No. 4,002,068 (Borst); U.S. Pat. No. 4,006,636 (Holmen); U.S.Pat. No. 4,422,327 (Anderson); U.S. Pat. No. 4,567,761 (Fajeau); U.S.Pat. No. 4,669,309 (Cornelius); U.S. Pat. No. 4,711,127 (Häfner); U.S.Pat. No. 5,005,408 (Glassey); U.S. Pat. No. 5,052,222 (Stoepfel); U.S.Pat. No. 5,090,242 (Hilton); U.S. Pat. No. 5,146,783 (Jansche et al.);U.S. Pat. No. 5,167,144 (Schneider); U.S. Pat. No. 5,207,251 (Cooks);U.S. Pat. No. 5,309,764 (Waldrop et al.); U.S. Pat. No. 5,406,828(Hunter et al.); U.S. Pat. No. 5,517,869 (Vories); U.S. Pat. No.5,636,547 (Raj et al.); U.S. Pat. No. 5,650,561 (Tubergen); and U.S.Pat. No. 5,953,954 (Drain et al.). These patents show various types oflevel measuring apparatuses representing the state of the art. In thecases of bubble type apparatuses, many use pneumatic tubes havingdifferent lengths to carry out differential pressure measurements. Suchdifferential measurements have their advantages but nevertheless do notsolve the problems associated with the sensitivity coefficient and theoffset factor of the pressure sensors used in the apparatuses.

SUMMARY

[0008] An object of the invention is to provide a bubble level meterwhich allows detection and monitoring of changes at the level of thesensitivity coefficient of the pressure sensor used by the apparatus, tofully eliminate or else reduce the doubts and errors in the readings ofthe apparatus caused by such changes.

[0009] Another object of the invention is to provide such a level meterwhich allows establishing a new sensitivity coefficient for the pressuresensor, during use of the level meter.

[0010] Another object of the invention is to provide such a level meterwhich may correct the offset factor of the pressure sensor.

[0011] Another object of the invention is to provide a method by which aconventional bubble level meter can be adapted to determine thesensitivity coefficient and the offset factor of the pressure sensorused by the apparatus and to correct the readings of the apparatus.

[0012] According to the present invention, there is provided a bubblelevel meter comprising:

[0013] a pneumatic tube submersible in part and having opposite lowerand upper openings;

[0014] a gas generator connected to the upper opening of the pneumatictube;

[0015] a pressure sensor connected to the upper opening of the pneumatictube in order to measure a pressure in the pneumatic tube;

[0016] a deflection valve interposed along the pneumatic tube above andat a predetermined distance from the lower opening, the deflection valvehaving a port for communication with an external liquid milieu in whicha submerged portion of the pneumatic tube is located, and closed andopen positions wherein the upper opening of the pneumatic tubecommunicates respectively with the lower opening of the pneumatic tubeand the port of the deflection valve; and

[0017] a control circuit connected to the gas generator, the pressuresensor and the deflecting valve, the control circuit being configuredfor:

[0018] processing pressure measurements obtained from the pressuresensor;

[0019] controlling the deflection valve and the gas generator as afunction of preset settings;

[0020] verifying a calibration coefficient of the pressure sensor as afunction of the pressure measurements obtained from the pressure sensorwhen the deflection valve is alternately in closed position and in openposition, and as a function of the distance between the lower opening ofthe pneumatic tube and the port of the deflection valve; and

[0021] generating level data as a function of the processed pressuremeasurements and the calibration coefficient.

[0022] Preferably, the level meter will further comprise:

[0023] an additional deflection valve interposed between the pressuresensor and the upper opening of the pneumatic tube, the additionaldeflection valve being connected to the control circuit and having aport for communication with an external atmospheric milieu in which anemerged portion of the pneumatic tube is located, and closed and openpositions wherein the pressure sensor communicates respectively with theupper opening of the pneumatic tube and the port of the additionaldeflection valve;

[0024] and wherein the control circuit is also configured for:

[0025] controlling the additional deflection valve as function of thepreset settings; and

[0026] verifying an offset factor of the pressure sensor as a functionof the pressure measurements obtained from the pressure sensor when theadditional deflection valve is alternately in closed position and inopen position, the level data generated by the control circuit being

[0027] also as a function of the offset factor.

[0028] According to the present invention, there is also provided amethod for improving pressure measurements in a bubble level metercomprising a pneumatic tube submersible in part having opposite lowerand upper openings, a gas generator connected to the upper opening ofthe pneumatic tube, a pressure sensor connected to the upper opening ofthe pneumatic tube, and a control circuit connected to the gas generatorand the pressure sensor and configured for processing measurementsobtained from the pressure sensor and generating level data as afunction of the processed measurements, the method comprising:

[0029] interposing a deflection valve along the pneumatic tube above andat a predetermined distance from the lower opening, the deflection valvehaving a port for communication with an external liquid milieu in whicha submerged portion of the pneumatic tube is located, and closed andopen positions wherein the upper opening of the pneumatic tubecommunicates respectively with the lower opening of the pneumatic tubeand the port of the deflection valve;

[0030] connecting the deflection valve to the control circuit; and

[0031] configuring the control circuit for:

[0032] controlling the deflection valve as a function of presetsettings;

[0033] verifying a calibration coefficient of the pressure sensor as afunction of pressure measurements obtained from the pressure sensor whenthe deflection valve is alternately in closed position and in openposition and as a function of the distance between the lower opening ofthe pneumatic tube and the port of the deflection valve; and

[0034] generating the level data as a function of the calibrationcoefficient.

[0035] Preferably, the method further comprises:

[0036] interposing an additional deflection valve between the pressuresensor and the upper opening of the pneumatic tube, the additionaldeflection valve having a port for communication with an externalatmospheric milieu in which an emerged portion of the pneumatic tube islocated, and closed and open positions wherein the pressure sensorcommunicates respectively with the upper opening of the pneumatic tubeand the port of the additional deflection valve;

[0037] connecting the additional valve to the control circuit; and

[0038] configuring the control circuit for:

[0039] controlling the additional deflection valve as a function of thepreset settings;

[0040] verifying an offset factor of the pressure sensor as a functionof the pressure measurements obtained from the pressure sensor when theadditional deflection valve is alternately in closed position and inopen position; and

[0041] generating the level data as a function of the offset factor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] A detailed description of the preferred embodiments of theinvention will be given hereinafter in reference with the followingdrawings, wherein the same reference numerals refer to identical orsimilar elements:

[0043]FIG. 1 is a schematic diagram of a bubble level meter according tothe invention; and

[0044] FIGS. 2 to 11 are tables and graphs illustrating examples ofcalculations of water levels, calibration coefficients and offsetfactors under different possible conditions of the level meter accordingto the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Referring to FIG. 1, there is shown a bubble level meteraccording to the invention. The level meter comprises a pneumatic tube 2intended to be partially immerged in a body of water 6, e.g. a lake or areservoir, or any other liquid whose level is to be measured or to bemonitored. The pneumatic tube 2 has opposite lower and upper openings 8,10. A gas generator 12, e.g. of air, is connected to the upper opening10 of the tube 2. A pressure sensor 14 is also connected to the upperopening 10 of the tube, in order to measure a pressure in the pneumatictube 2.

[0046] A deflection valve 16 is interposed along the pneumatic tube 2above and at a predetermined distance from the lower opening 8. Thedeflection valve 16 has a port 18 for communication with an externalliquid milieu, e.g. the water 6, in which a submerged portion 9 of thepneumatic tube 2 is located, and closed and open positions wherein theupper opening 10 of the tube 2 communicates respectively with the loweropening 8 and the port 18 of the valve 16. The function of thedeflection valve 16 is to allow verification and optionally acorrection/update of the calibration coefficient(s) of the pressuresensor 14. More details concerning the operation mode of the deflectionvalve 16 are provided hereinafter.

[0047] A control circuit 20 is connected to the gas generator 12, thepressure sensor 14 and the deflection valve 16, for example throughlines 22, 24, 26 respectively. The control circuit 20 is configured forprocessing pressure measurements obtained from the pressure sensor 14,controlling the deflection valve 16 and the gas generator 12 as afunction of preset settings, verifying the calibration coefficient ofthe pressure sensor 14 as a function of the pressure measurementsobtained from the sensor 14 when the deflection valve 16 is alternatelyin closed position and in open position and as a function of thedistance F between the lower opening 8 of the pneumatic tube 2 and theport 18 of the deflection valve 16. The control circuit 20 is alsoconfigured for generating level data as a function of the processedpressure measurements and the calibration coefficient. More informationon the operation of the circuit 20 is provided hereinafter.

[0048] An additional deflection valve 28 is preferably interposedbetween the pressure sensor 14 and the upper opening 10 of the pneumatictube 2. The valve 28 is connected to the control circuit 20 for examplethrough the line 30, and has a port 32 for communication with anexternal atmospheric milieu, e.g. ambient air, in which an emergedportion 11 of the pneumatic tube 2 is located, and closed and openpositions wherein the pressure sensor 14 communicates respectively withthe upper opening 10 of the tube 2 and the port 32 of the valve 28. Thefunction of the deflection valve 28 is to allow verification andoptionally a correction/update of the offset factor of the pressuresensor 14. More details regarding the operation mode of the deflectionvalve 28 are provided hereinafter. The control circuit 20 is configuredfor controlling the additional deflection valve 28 in the same way asfor the other valve 16, and for verifying the offset factor of thesensor 14 as a function of the pressure measurements obtained from thesensor 14 when the valve 28 is alternately in closed position and inopen position. The control circuit 20 then takes the offset factor intoaccount when generating the level data.

[0049] The on-site assembly and installation of the main components ofthe apparatus are simple and require little space. To obtain ameasurement of the water level G with the level meter, the pneumatictube 2 must be immerged into the water. The tube 2 is lowered at theminimum elevation C to be measured in the body or reservoir of water 6.Preferably, the tube 2 is secured to the walls of the body or reservoirof water 6. The lower end reference of the tube 2 is important foraccurate results on the final elevation of the measurement. The lowerend of the tube 2 should be free from any object or other element whichcould block the air outlet 8.

[0050] In certain cases where hostile environment prevails, the tube 2will preferably be protected by a sheath or a protection tube 4,preventing an object or debris to damage or squeeze the tube 2. Thepneumatic tube 2 can be fastened to the protection tube 4 by means ofattachments 34 while the protection tube 4 can be secured to the wallsof the body of water by means of other attachments 36. Preferably, theprotection tube 4 should not exceed the lower end of the pneumatic tube2.

[0051] The upper opening 10 of the pneumatic tube 2 is then connected tothe air generator 12. The pressure produced by the air generator 12 mustbe sufficient for opposing the water pressure which is exerted at thelower end of the pneumatic tube 2.

[0052] The pneumatic tube 2 and the air generator 12 are connected tothe pressure sensor 14 which can be electrical, electronic, pneumatic,or of another type. The pressure sensor 14 will preferably beintegrated, along with electronic control components schematized by thecontrol circuit 20, to the level meter. With these elements soassembled, it becomes possible to obtain a reading of the pressure whichis exerted at the lower end of the pneumatic tube 2.

[0053] The pressure of air simultaneously applied on the pressure sensor14 will be determinable with a very high accuracy by injection of apressure of air equal to or balanced with the pressure exerted by thewater column G above the lower opening 8 of the pneumatic tube 2. Theprecise moment of balance between the water pressure at the loweropening 8 of the pneumatic tube 2 and the pressure of air injected inthe pneumatic tube 2, represents the measurement of the water level A.This pressure will be transformed afterwards in a water height G locatedabove the lower opening 8 forming the air outlet of the pneumatic tube2.

[0054] An advantage of the lever meter resides in the fact that all theprecision measurement components are outside the body or reservoir ofwater 6 to be measured. The pressure sensor 14, the electronic controlcomponents 20, the air generator 12, etc., are located outside a milieuwhich is very often hostile to the various components which constitutethe level meter. Thus, it then becomes possible to control the elementswhich generally impair the stability of these components, such as thetemperature, humidity, corrosion, vibration, etc., which may affect thefinal accuracy of the measurement of the level A. It is also veryadvantageous to have the components outside the water 6 for theirmaintenance and their repair if need be.

[0055] Cancellation of the offset in time of the pressure sensor 14 isachieved by subjecting the sensor 14 to free air D before each pressuremeasurement operation. By measuring this new reading of the offset atfree air D, the difference which may appear with respect to the initialoffset of the sensor 14 can be mathematically and electronicallycancelled or even better, this offset can be subtracted from the finalreading of the pressure measured by the sensor 14.

[0056] Regarding the calibration of the sensor 14, it consists of usingthe water column F which will be pre-established and maintained duringthe use of the level meter. It must be understood that this water columnF, known by the user at the time of the on-site installation of theapparatus, represents also a distance F between two precise points B, Clocated inside the body or reservoir of water 6. This distance Fconverted in water pressure, determined in advance during the initialinstallation of the apparatus, allows to verify if the measured pressureis correct as a function of the original sensitivity coefficient of thepressure sensor 14.

[0057] The proposed method allows the user to verify if, for any reason,the pressure sensor 14 has maintained its original sensitivitycoefficient. By means of this method, the user can also know, withprecision, the errors on levels A caused by the sensitivity coefficient.This represents a major trump for bubble level meters. Thus, thenecessary corrections can be achieved so as to obtain a precise andaccurate measurement of the level A.

[0058] To achieve this important verification, the principle is totemporarily deflect the air from the lower end of the outlet 8 of thepneumatic tube 2 by means of the bi-directional solenoid valve 16 or anyother device which provides the same result. The solenoid valve 16fulfills two important functions, namely that in closed position, theair injected inside the pneumatic tube 2 fully and only goes to theoutlet 8 of the pneumatic tube 2 (normal operating principle of thebubble level meter) and that in open position, the air injected insidethe pneumatic tube 2 is deflected totally and only at the installationlevel B of the solenoid valve 16.

[0059] The solenoid valve 16 allows to obtain a fixed reference waterpressure column F. This distance F fixed between two different airoutlet points B, C implies that the pressure is always identical betweenthese two measurement points provided that the water level A remains thesame during the reading of these two measurement points. By deflectingthe air outlet 8 at a known distance F, it becomes possible to verifythe behaviour of the pressure sensor 14 for a distance F already knownand established during the installation of the apparatus. Since thedistance F between the air outlet 8 with respect to the outlet 18 of airdeflected at the solenoid valve 16 is known, the measurement F can beassociated to a corresponding pressure.

[0060] This method allows to verify with great accuracy that theoriginal sensitivity coefficient of the pressure sensor 14 is valid.This verification corresponds to a single level of pressure applied onthe pressure sensor 14. With this method, it is assumed that thepressure sensor 14 is linear over the totality of its full range. Thelinearity of a pressure measurement sensor remains generally very good,except of course when its measurement diaphragm has been deformed ordamaged by pressures exceeding its full measurement range. Since thisapproach confirms a single reference point on the original calibrationcurve of the sensor 14, the same air deflection principle can be appliedon several different elevation levels, with only one pneumatic tube 2.It then becomes possible to verify if the sensor 14 is linear for itsfull measurement scale.

[0061] With the positioning of the solenoid valve 16, it is possible, inaddition to verifying that the original sensitivity coefficient of thesensor 14 has remained unimpaired, to establish in the oppositesituation, a new sensitivity coefficient for the sensor 14. This sameverification method allows also to determine the pressure or water levelheight reading error caused by a variation of the sensitivitycoefficient. The air deflection on a same pneumatic tube 2 allows theuser to verify and to know, with accuracy and at all times, the errorsproduced on the measurement of the water level A.

[0062] The method of verifying and correcting the sensitivitycoefficient of the pressure sensor 14, combined with the possiblecorrection of the offset of the sensor 14, is a simple, efficient andcostless means to ascertain the validity of the reading of the sensor 14with all its components located inside the bubble level meter.

[0063] The control circuit 20 can be provided with a memory for storingthe level data for later processing, and the settings and/or operationparameters of the circuit 20. The control circuit can also be providedwith a display device 46 for displaying for example the measurementresults, the operating parameters and modes of the level meter, etc. Thesensitive components of the level meter such as the gas generator 12,the pressure sensor 14 and the control circuit 20 can be disposed in anenclosure 44 for protecting them from bad weather. The outlet 32 of thevalve 28 can then be connected to a tube 48 exiting a lateral opening 50of the enclosure and leading to the external atmospheric milieu, whereasthe pneumatic tube 2 can be inserted in a lower opening 54 of theenclosure to communicate with the gas generator 12 and the pressuresensor 14.

[0064] FIGS. 2 to 11 provide examples of calculations of water levels,of calibration coefficients and of offset factors under differentpossible conditions of the level meter according to the invention. FIG.2 shows simulated typical data when there is no offset and no variationof the calibration coefficient of the sensor 14. FIG. 3 shows theresults obtained with (line with marks) and without (line without marks)verification of the offset and of the calibration coefficient of thesensor 14 under this condition. FIG. 4 shows simulated typical data whenthere is a possible variation of the offset of the sensor 14 only. FIG.5 illustrates the results obtained with (line with marks) and without(line without marks) verification of the offset and of the calibrationcoefficient of the sensor 14 under this condition. FIG. 6 showssimulated typical data when there is a possible variation of thecalibration coefficient of the sensor 14 only. FIG. 7 illustrates theresults obtained with (line with marks) and without (line without marks)verification of the offset and of the calibration coefficient of thesensor 14 under this condition. FIGS. 8 and 10 show simulated typicaldata when there are two types of possible variations of the offset andof the calibration coefficient of the sensor 14. FIGS. 9 and 11illustrate the results obtained with (line with marks) and without (linewithout marks) verification of the offset and of the calibrationcoefficient of the sensor 14 under these conditions.

[0065] The parameters generally established by the manufacturer of thepressure sensor 14 are: the initial full scale P_(E) of the sensor (inmeters); the initial sensitivity S_(O) of the sensor at full scale (inmV/V); the initial calibration coefficient K_(M) _(O) (in meters/mV/V);and the initial calibration coefficient K_(V) _(O) (in mV/V/meter).These basic data are initially integrated into the level meter.

[0066] The efficiency of the sensor 14 is directly related to the finalaccuracy on the measurement of level A. The choice of the fullmeasurement scale of the sensor is also determinant with respect to thedesired sensitivity over the variations of the water level A. It isnecessary to ensure having a good ratio between the full measurementscale of the sensor 14 and the maximum level to be measured. If fordifferent circumstances this ratio is too low, the final accuracy overthe variation of the water level A is likely to be affected. Of course,all the associated components which supply or control the sensor 14 mustbe chosen so as to not reduce its accuracy. The electronic componentssuch as: power supply, analog to digital converter, digital to analogconverter, etc., must preferably correspond to selection criteria whichfollow closely the original characteristics of the pressure sensor 14.

[0067] The elevations E_(A) _(O) , E_(B) _(O) and E_(C) _(O) (in meters)of the water level, of the solenoid valve 16 and of the outlet 8 of thetube 2, respectively, must be measured during the on-site installationof the level meter. These initial elevations of the measurement points,namely the elevation B of the solenoid valve 16 and the elevation C ofthe lower end 8 of the pneumatic tube 2 are at the base of thesubsequent calculations, in order to obtain an exact reference on thewater level A which is to be measured. These initial parameters shouldpreferably be obtained with an accurate geodesic measurement system orany other system which will be capable of relating the initialelevations of the two measurement points B, C to a reference elevationalready established.

[0068] These elevation references of the measurement points B, C mustnot change at any time after the installation of the apparatus. Theseelevations B, C are the references to which the heights E, G of themeasured water columns will be respectively added in order to obtain thefinal elevation A of the body of water 6. Any variation of the elevationB, C could cause an error on the final result of the reading of thewater elevation A as well as on the application of the correction of thesensitivity coefficient of the pressure sensor 14.

[0069] As previously mentioned, the choice of the full scale of thepressure sensor 14 is determinant for the final sensitivity of theresults. The initial positioning of the air outlet 8 at the lower end ofthe pneumatic tube 2 will thus be in direct relation with this choice offull measurement scale of the sensor 14. The position or elevation C ofthe lower end of the tube 2 indicates the reference to which themeasured water height G which is located above this lower end of thetube 2 will be added. The choice of the elevation B of the solenoidvalve 16 is determinant for the application of the method of airdeflection located at the solenoid valve 16. The major and criticalpoint in the application of the method is that the final elevation B ofthe solenoid valve 16 must remain at all times lower than the minimalelevation of the water body which will be measured. The air deflectionsystem of the solenoid valve 16 becomes dysfunctional in this particularcase. It is important to properly foresee this situation during theinitial installation of the measurement system.

[0070] In order that the verification of the sensitivity coefficient ofthe pressure sensor 14 be the most efficient as possible, the solenoidvalve 16 must preferably be installed as close as possible to theminimal elevation of the body of water to be measured. When soinstalled, there is obtained an excellent efficiency ratio with respectto the full scale of the pressure sensor 14. It must be well understoodthat the installation distance F between the air outlet 8 and the airoutlet 18 represents the pressure reading which will be compared toverify the sensitivity coefficient of the pressure sensor 14. Thus, themore the distance F between these two measurement points will beimportant, the more the pressure reading will be representative of thefull scale of the pressure sensor 14.

[0071] It is also possible with the air deflection method to add asecond solenoid valve (not illustrated), in order to have a shorterdistance with respect to the lower end of the outlet 8 of the pneumatictube 2. Thus, it could be possible to verify the sensitivity coefficientof the pressure sensor 14 at minimal and maximal pressures of the fullscale of the sensor 14.

[0072] The elevation D of the free air outlet 32 which is in directrelation with the offset factor of the pressure sensor 14, can beconsidered as a reference value for the apparatus, since many solutionsare possible to correct this offset error. One of the most efficientways to cancel this offset of the pressure sensor 14 is to subject thissame sensor 14 to free air D, by means of the bidirectional solenoidvalve 28. This method ensures that the sensor 14 is subjected to nopressure except that of the ambient air where the apparatus is located.With this method, it is ensured that the pressure sensor 14 measures aninitial offset which is only caused either by the change of atmosphericpressure or by components connected to this same pressure sensor 14.

[0073] Many elements, other than the atmospheric pressure, maycontribute to the total offset of a level meter. These offset elementsare often related to residual mechanical constraints of the pressuresensor 14, of the electric or electronic components 20 of the wholemeasurement system which are more or less steady with the operationtime, and numerous other non-negligible points to obtain an accuratepressure measurement. Even if it is possible to mathematically cancelthis offset with the free air deflection solenoid valve 28, it is goodto know the initial offset of the pressure sensor 14. This allows toverify if the pressure sensor 14 or the other auxiliary componentsconnected to the pressure sensor 14 have undergone degradation duringthe lifetime of the apparatus.

[0074] During the installation of the level meter, the initial readingsof the pressure sensor 14 are important references which are used toverify that the sensor 14 properly operates at the time of itsinstallation. The readings L_(D) _(O) , L_(B) _(O) and L_(C) _(O) (inmV/V) at free air, at the outlet of the solenoid valve 16 and at theoutlet 8 of the tube 2 are respectively taken for this purpose.

[0075] The initial reading at free air D can be optional in the casewhere the apparatus is deprived of a valve 28. It must be, in thisparticular case, assumed that the offset of the sensor 14 and of theauxiliary components will be identical for the whole lifetime of theapparatus. In the case where the pressure sensor 14 is subjected to freeair D before each measurement operation, and that the reading of theoffset so obtained is corrected from the final reading of the sensor 14,this offset reading becomes a highly important reference for theapplication of the following equations.

[0076] The initial on-site reading of the offset is also important forfollowing up the total offset of the measurement system during the wholeinstallation lifetime of the apparatus. It is the suggested andrecommended solution.

[0077] For properly verifying the sensitivity coefficient of thepressure sensor 14, it is recommended to have a quasi-instantaneousreading between the outlet 8 of the lower end of the pneumatic tube 2and the air deflection 18 of the solenoid valve 16. The logic sequencefor optimizing a measurement with the control circuit or microprocessor20 is important. In verification mode, the microprocessor 20 actuatesthe solenoid valve 28 of the free air outlet 32 in open position. Atthis precise moment, the solenoid valve 16 of the air deflection 42 ofthe pneumatic tube 2 must be in closed position. This allows to maintainthe full and existing pressure inside the pneumatic tube 2. After anoffset reading of the pressure sensor 14, the microprocessor 20 actuatesthe solenoid valve 28 in closed position. Thereby, it will be possibleto obtain a pressure reading at the lower end of the pneumatic tube 2.Finally, the microprocessor 20 actuates the solenoid valve 16 in openposition. This last action allows to obtain the pressure of the waterlocated above the tube outlet 38 of the air deviation connected to theoutlet 18 of the solenoid valve 16. After this last sequence, themicroprocessor 20 sets the solenoid valve 16 back in closed position.This resets the measurement apparatus in normal reading position.

[0078] A reading at free air LD is preferably achieved before thereading of the two measurement points LB and Lc. With the followingequations, it is assumed that the offset measured at the pressure sensor14 is not different between the time of the reading located at the lowerend C and the air deflection B. The elevation of the water level A (inmeters) from the readings taken at the levels B and C can be calculatedby:

E _(A) _(B,C) =(L _(B,C) =L _(D) _(O) )×K _(M) _(O) +E _(B) _(O) _(,C)_(O)

[0079] An important point during the measurement at B and at C is thatthe water level A must preferably remain as steady as possible duringthis measurement period for having a good accuracy during themathematical verification of the sensitivity coefficient. A fastexecution between the readings is necessary.

[0080] Once two readings of the water level A have been obtained, it ispossible to immediately compare the results to detect if the originalsensitivity coefficient of the pressure sensor 14 has changed. The delta(in meters) caused by the variation of the calibration coefficientbetween the two calculated water levels E_(A) _(B) and E_(A) _(C) iscalculated by:

ΔE _(A) _(BC) =E _(A) _(B) −E _(A) _(C) =(L _(B) −L _(C))×K_(M) _(O) +E_(B) _(O) −E _(C) _(O)

[0081] In the case where the difference obtained between the twomeasured elevations is equal to zero, it can be concluded that theoriginal sensitivity coefficient of the pressure sensor 14 has remainedexact. Conversely, if there is obtained a difference between the twomeasured elevations non-equal to zero, it can be deduced or affirmedthat the pressure sensor 14 has a more or less significant error on itsoriginal sensitivity coefficient. This value does not represent, at thismoment, an error in meters on the final result of the elevation A of thelevel measurement.

[0082] Mathematically, it can also be affirmed that the level readingdifference between the lower end of the outlet 8 of the pneumatic tube 2and the air deflection 18 of the solenoid valve 16 must correspond tothe difference of the original elevation of the on-site installation B,C of the two same air outlets 8, 18 on the pneumatic tube 2.

[0083] With these verifications, it is possible to correct thesensitivity coefficient with the following formulas.

[0084] Following the verification of the sensitivity coefficient, theoffset of the pressure sensor 14 is verified and calculated should thiserror must be taken into account in the next calculations. This can beachieved by:

[0085] ΔD=D−D₀=L_(D)−L_(D) _(O) in mV/V with D₀=L_(D) _(O) representingthe initial offset of the sensor, in mV/V, and D=L_(D) representing theon-site offset, in mV/V.

[0086] With this calculation method, a follow-up of the offset sinceon-site installation of the sensor 14 is ensured, or even better, theevolution of the offset since manufacture of the apparatus can bemonitored. The subsequent equations indicate a correction of the offsetsince the on-site installation of the apparatus.

[0087] In the case where the difference between the on-site originaloffset reading and the actual offset reading is equal to zero, thisindicates that the pressure sensor 14 as well as all the other relatedor auxiliary components connected to the sensor 14 have remainedidentical.

[0088] Conversely, if there is obtained a value different from zero, itcan be affirmed that the sensor 14 has a more or less significant erroron the final result of the measurement of the water level A. This valuerepresents at this moment a difference in mV/V in excess to or under thefinal result of elevation of the measurement of the water level A.

[0089] With the previous verifications made, it becomes possible toassociate a new sensitivity coefficient for the pressure sensor 14, tocorrect the offset error and to know all the errors related to these twoprecision phenomenon of the apparatus. The calculation of the newcalibration coefficient can be achieved using the formulas:${S = {P_{E} \times \frac{\left( {L_{C} - L_{B}} \right)}{\left( {E_{B_{O}} - E_{C_{O}}} \right)}}},$

[0090] S representing the calculated sensitivity of the sensor on thefull scale, in mV/V;${K_{M} = {\frac{P_{E}}{S} = \frac{\left( {E_{B_{O}} - E_{C_{O}}} \right)}{\left( {L_{C} - L_{B}} \right)}}},$

[0091] K_(M) representing the calculated calibration coefficient, inmeters/mV/V; and ${K_{V} = \frac{1}{K_{M}}},$

[0092] K_(V) representing the calculated calibration coefficient, inmV/V/meter.

[0093] With these equations, it is possible to calculate a newsensitivity coefficient for the pressure sensor 14 with respect to itsfull measurement scale. These equations are based on the fact that therelation between the difference of the elevation C of the lower end ofthe outlet 8 of the pneumatic tube 2 and the elevation B of the airdeflections 18 of the solenoid valve 16 as well as the differencebetween the obtained reading of the level A at the two measurementpoints should be identical.

[0094] The other calibration coefficients can be calculated based on thesame principle. It is possible to use only the new sensitivitycoefficients to determine the other calibration coefficients. It isgenerally this method which is used by the manufacturers of pressuresensors.

[0095] With this new sensitivity coefficient and the other calibrationcoefficients, the original coefficients of the apparatus can be replacedif more accurate and precise subsequent readings of the level A aredesired. Only the value of the full scale of the pressure sensor 14remains the same at all times.

[0096] The error caused by the offset of the sensor 14 can be easilyexpressed in water meters. The following equations take the originalsensitivity coefficient of the pressure sensor 14 into account. With thefollowing equations, it is possible to calculate the error in meters dueto an offset of the pressure sensor 14. This error in meter is based onthe principle of the verification established on-site between thereading of the free air outlet D and the initial reading of the offsetof the sensor 14:

C _(D) _(A) , C _(D) _(B) , C _(D) _(C) =ΔD×K _(M) _(O) =(L _(D,B,C) −L_(D) _(O) )×K _(M) _(O) , C _(D) _(A) , C _(D) _(B) , C _(D) _(C)

[0097] representing the error calculations of the water level A causedby the offset of the sensor with readings at free air D and atelevations B and C, respectively (in meters).

[0098] The previous equations establish an error in meters related tothe original sensitivity coefficient. The error is presumed identicalfor both measurement points B, C since only one reference reading atfree air D has been carried out (a positive value indicates a risingwater level). It is also assumed that the sensitivity coefficient hasremained the same between the time of the measurement at free air D andthe other measurement points B, C.

[0099] It is also possible to mathematically find the error of level Aof each measurement due to an erroneous sensitivity coefficient. Withthe following equations, it is possible to determine with accuracy thedeviation of the level A measured at the lower end of the outlet 8 ofthe pneumatic tube 2 as well as at the air outlet 18 deflected by meansof the solenoid valve 16 (a positive value indicates a rising waterlevel):${C_{K_{B}} = {\left( {E_{A_{B}} - C_{D_{B}} - E_{B_{O}}} \right) \times \frac{\left( {E_{A_{C}} - E_{A_{B}}} \right)}{\left( {\left( {E_{A_{C}} - E_{A_{B}}} \right) - \left( {E_{C_{O}} - E_{B_{O}}} \right)} \right)}}},$

[0100] C_(K) _(B) representing the calculation of the error of the waterlevel A caused by the variation of the calibration coefficientcalculated with a reading in B, in meters; and${C_{K_{C}} = {\left( {E_{A_{C}} - C_{D_{C}} - E_{C_{O}}} \right) \times \frac{\left( {E_{A_{C}} - E_{A_{B}}} \right)}{\left( {\left( {E_{A_{C}} - E_{A_{B}}} \right) - \left( {E_{C_{O}} - E_{B_{O}}} \right)} \right)}}},$

[0101] C_(K) _(C) representing the calculation of the error of the waterlevel A caused by the variation of the calibration coefficientcalculated with a reading in C, in meters.

[0102] These readings take the offset of the pressure sensor 14 intoaccount. The error produced by the calculation is only due to a changeof the sensitivity coefficient of the sensor 14. The reading representsthe variation which must be taken into account by a user in his actualreading if the original sensitivity coefficient has not been replaced bythe new calibration sensitivity coefficient found previously in theequations.

[0103] It is finally possible to find an elevation of the water level Acorrected by taking both possible error variations into account, namelythe change of the sensitivity coefficient and the offset factor of thepressure sensor 14:

[0104] E_(A) _(BN,CN) =E_(A) _(B,C) −C_(K) _(B,C) −C_(D) _(B,C) , E_(A)_(BN,CN) representing the elevation of the water level A corrected dueto the offset of the sensor and/or to the variation of the calibrationcoefficient, calculated with a reading in B and in C, respectively, inmeters.

[0105] The equation takes both individual corrections into account inorder to indicate the proper measurement elevation of the water level A.These readings represent the result of the final and corrected elevationof the water A, in the case where the original sensitivity coefficienthas not been replaced by the new calibration sensitivity coefficient.The offset factor error of the pressure sensor 14 is maintainedthroughout this process.

[0106] The apparatus may comprise a second measurement sensor with itsown pneumatic linking tube having the same characteristics as theprimary sensor (not illustrated). The purpose of this second measurementpoint is essentially to validate the reading of the primary sensor. Theoperator may insert, if desired, a level difference or acceptable gaugebetween the readings of the sensors, thereby ensuring a certain validityof the reading in the case where a reading would appear doubtful. Thisprocess can be managed by the apparatus itself, indicating then to theoperator if the reading is good or doubtful.

[0107] The apparatus thus allows to have accurate and reliable levelreadings, to minimize the long-term maintenance cost of the measurementapparatus, to minimize the installation and handling costs of themeasurement apparatus. The apparatus may have a hardware and softwaredesign responding to the technological criteria of the years 2000, andan excellent quality/price.

[0108] The apparatus will thereby be capable to correct, electronicallyor by preprogrammed calculation methods, all the imponderables which mayaffect the accuracy of the final reading.

[0109] The apparatus may be constructed so as to allow to manage itsautonomy through rechargeable cells at different operating conditionsrequired by the user and to manage the different calculation algorithmsrequired for its proper operation, to establish a good management of thestored data and parameters, to facilitate a proper communication with acomputer, to be user friendly overall. The apparatus may have a water,dust, oil, etc. -proof casing, have only one casing 44 in stainlesssteel or painted aluminum or PVC, have locks allowing the installationof padlocks (not illustrated), have access to different readings oflevels through a display window, have waterproof keyboard keys 52allowing the users to modify the basic data of the apparatus, havecompression glands for all the electrical cable inputs and for the inletand outlet tubes, have an alphanumeric display (LCD) 46 or the like topermanently indicate the variations and other parameters. It may belighted and heated if necessary. The complete apparatus should be ableto operate at temperatures varying from −55° to +60° C., and should beinstallable as well on a wall as on a pipe with a base. The measurementapparatus may, for example, be adapted for a 120 VAC power supply, maybe adapted to operate with a 12 VDC external cell, may allow a supplysource through solar panels or windmills, may be provided with all theconnectors or terminals for supply purposes (120 VAC, 12 VDC, solar andwindmill), may be provided with a 12 VDC output with screwed terminal,may have a 5 VDC output with a screwed terminal, may have a rechargeablecell capable to withstand a main supply failure for a minimal period ofseven (7) days as a function of the operation mode already establishedby the user, may have a low total consumption (mA) and be provided witha sleep mode device on different components established by the user, mayhave an automatic device allowing to reduce the taking of readings incase of power failure, may have an automatic alarm device in case ofpower failure, may have mechanical protecting circuit breakers on the120 VAC input as well as on the 12 VDC supply. The apparatus can beprovided with a compressed air generator capable to generate a properpressure for all the ranges of the measurement extents mentioned below.A manual device can also be provided in the event that the user wouldopt for nitrogen bottles in order to generate the pressure. Theapparatus may contain all the necessary couplings and valves so that theuser may chose himself/herself the operation mode which suits him/her(integrated air generator or nitrogen bottle).

[0110] An automatic pressure transfer device can be installed betweenthe compressed air generator and the nitrogen bottles in order todeactivate the pressure generator in the event that the state of therechargeable cells of the apparatus would have reached a criticalthreshold (Volt/Amperage). This device would be very useful inparticular with the power supply using solar panels or windmills. Thesame automatic pressure transfer device may be used in the event offailure of the air generator.

[0111] Many parameters can be measured in real time, such as the stateof the power supply in Volts, the state of the rechargeable cells inVolts, the internal temperature in ° C., the state of the air reserve inkPa, the state of validity of the sensors, their relative or absolutegauge in meters, the variation of the gauge in meters, the relativelevel in meters, the gradient on the variation speed in meters/mm, thepressure of the air reserve of the auxiliary nitrogen bottles in kPa,etc.

[0112] General operation parameters may be used, such as a versionnumber, a signature (random number representing the actual configurationof the apparatus), a positioning (longitude and latitude coordinates), alocation (upstream, downstream, body of water, etc.), a site(identification), an installation date, etc.

[0113] The design and the assembly of the different components may beachieved in modular fashion so that a quick maintenance of the apparatuscan be performed in the event of major or minor failures. The electroniccomponents can be assembled on a surface mount type printed circuit.

[0114] The basic apparatus can be arranged to communicate with a localcomputer system (site) and also to transmit data in a bi-directionalfashion with one or many central computers away from the site. All theseelements may be integral parts of the measurement apparatus.

[0115] With these features and technical performances of the levelmeter, the reliability of the whole arrangement of the measurementapparatus is significantly improved, in addition to improving thesecurity of the works and of the public.

[0116] Many existing level meters can be adapted according to theinvention, by adding the valve 16 on the tube 2 and by modifying theprogramming of the control circuit 20 or else by changing it forperforming the previously described verification mode. The valve 28 mayalso be added if necessary.

[0117] Although embodiments of the invention have been illustrated inthe attached drawings and described herein above, it will becomeapparent for persons skilled in the art that changes and modificationscan be made to these embodiments without departing from the invention.All such modifications or variants are considered to be within the scopeof the invention as defined in the following claims.

1. A bubble level meter, comprising: a pneumatic tube submersible inpart and having opposite lower and upper openings; a gas generatorconnected to the upper opening of the pneumatic tube; a pressure sensorconnected to the upper opening of the pneumatic tube in order to measurea pressure in the pneumatic tube; a deflection valve interposed alongthe pneumatic tube above and at a predetermined distance from the loweropening, the deflection valve having a port for communication with anexternal liquid milieu in which a submerged portion of the pneumatictube is located, and closed and open positions wherein the upper openingof the pneumatic tube communicates respectively with the lower openingof the pneumatic tube and the port of the deflection valve; and acontrol circuit connected to the gas generator, the pressure sensor andthe deflection valve, the control circuit being configured for:processing pressure measurements obtained from the pressure sensor;controlling the deflection valve and the gas generator as a function ofpreset settings; verifying a calibration coefficient of the pressuresensor as a function of the pressure measurements obtained from thepressure sensor when the deflection valve is alternately in closedposition and in open position, and as a function of the distance betweenthe lower opening of the pneumatic tube and the port of the deflectionvalve; and generating level data as a function of the processed pressuremeasurements and the calibration coefficient.
 2. The level meteraccording to claim 1, further comprising: an additional deflection valveinterposed between the pressure sensor and the upper opening of thepneumatic tube, the additional deflection valve being connected to thecontrol circuit and having a port for communication with an externalatmospheric milieu in which an emerged portion of the pneumatic tube islocated, and closed and open positions wherein the pressure sensorcommunicates respectively with the upper opening of the pneumatic tubeand the port of the additional deflection valve; and wherein the controlcircuit is further configured for: controlling the additional deflectionvalve as a function of the preset settings; and verifying an offsetfactor of the pressure sensor as a function of the pressure measurementsobtained from the pressure sensor when the additional deflection valveis alternately in closed position and in open position, the level datagenerated by the control circuit being also as a function of the offsetfactor.
 3. The level meter according to claim 2, wherein the controlcircuit comprises a memory for storing the level data.
 4. The levelmeter according to claim 3, wherein the preset settings are stored inthe memory.
 5. The level meter according to claim 2, wherein thedeflection valves comprise bi-directional solenoid valves under controlof the control circuit.
 6. The level meter according to claim 2, whereinthe pneumatic tube is provided with a protective tube surrounding thepneumatic tube over a length of the pneumatic tube.
 7. The level meteraccording to claim 2, further comprising an enclosure in which the gasgenerator, the pressure sensor and the control circuit are mounted, theenclosure having a lower opening receiving an upper end of the pneumatictube comprising the upper opening, and a lateral opening receiving atube connecting the port of the additional deflection valve with theexternal atmospheric milieu.
 8. The level meter according to claim 1,wherein the control circuit has: a normal measurement mode wherein thecontrol circuit sets the deflection valve in closed position and the gasgenerator in operation for opposing a hydric pressure in the pneumatictube until obtaining a balanced pressure at the lower opening of thepneumatic tube, and takes a pressure measurement from the pressuresensor when the balanced pressure is reached; and a verification modewherein the control circuit sets the deflection valve along thepneumatic tube in open position for a preset duration, takes a pressuremeasurement from the pressure sensor at the opening of the deflectionvalve, and determines the calibration coefficient as a function of thepressure measurement.
 9. The level meter according to claim 2, whereinthe control circuit has: a normal measurement mode wherein the controlcircuit sets the deflection valves in closed position and the gasgenerator in operation for opposing a hydric pressure inside thepneumatic tube until obtaining a balanced pressure at the lower openingof the pneumatic tube, and takes a pressure measurement from thepressure sensor when the balanced pressure is reached; a firstverification mode wherein the control circuit sets the deflection valvealong the pneumatic tube in open position for a preset duration, takes apressure measurement from the pressure sensor at the opening of thedeflection valve, and determines the calibration coefficient as afunction of the pressure measurement; and a second verification modewherein the control circuit sets the additional deflection valve betweenthe upper opening of the pneumatic tube and the pressure sensor in openposition for a preset duration, takes a pressure measurement from thepressure sensor at the opening of the additional deflection valve, anddetermines the offset factor as a function of the pressure measurement.10. The level meter according to claim 9, wherein, in the firstverification mode, the control circuit ensures that the additionaldeflection valve between the upper opening of the pneumatic tube and thepressure sensor is in closed position before the opening of thedeflection valve along the pneumatic tube, and in the secondverification mode, the control circuit ensures that the deflection valvealong the pneumatic tube is in closed position before the opening of theadditional deflection valve between the upper opening of the pneumatictube and the pressure sensor.
 11. The level meter according to claim 2,wherein the control circuit is provided with a display.
 12. The levelmeter according to claim 1, further comprising a supplementarydeflection valve interposed along the pneumatic tube between the loweropening of the pneumatic tube and the deflection valve already in place,the supplementary deflection valve having a port for communication withthe external liquid milieu, and closed and open positions wherein theupper opening of the pneumatic tube communicates respectively with thelower opening of the pneumatic tube and the port of the deflectionvalve, the control circuit being configured for operating the deflectionvalves independently.
 13. A method for improving pressure measurementsin a bubble level meter comprising a pneumatic tube submersible in parthaving opposite lower and upper openings, a gas generator connected tothe upper opening of the pneumatic tube, a pressure sensor connected tothe upper opening of the pneumatic tube, and a control circuit connectedto the gas generator and the pressure sensor and configured forprocessing measurements obtained from the pressure sensor and generatinglevel data as a function of the processed measurements, the methodcomprising: interposing a deflection valve along the pneumatic tubeabove and at a predetermined distance from the lower opening, thedeflection valve having a port for communication with an external liquidmilieu in which a submerged portion of the pneumatic tube is located,and closed and open positions wherein the upper opening of the pneumatictube communicates respectively with the lower opening of the pneumatictube and the port of the deflection valve; connecting the deflectionvalve to the control circuit; and configuring the control circuit for:controlling the deflection valve as a function of preset settings;verifying a calibration coefficient of the pressure sensor as a functionof pressure measurements obtained from the pressure sensor when thedeflection valve is alternately in closed position and in open position,and as a function of the distance between the lower opening of thepneumatic tube and the port of the deflection valve; and generating thelevel data as a function of the calibration coefficient.
 14. The methodaccording to claim 13, further comprising: interposing an additionaldeflection valve between the pressure sensor and the upper opening ofthe pneumatic tube, the additional deflection valve having a port forcommunication with an external atmospheric milieu in which an emergedportion of the pneumatic tube is located, and closed and open positionswherein the pressure sensor communicates respectively with the upperopening of the pneumatic tube and the port of the additional deflectionvalve; connecting the additional deflection valve to the controlcircuit; and configuring the control circuit for: controlling theadditional deflection valve as a function of the preset settings;verifying an offset factor of the pressure sensor as a function of thepressure measurements obtained from the pressure sensor when theadditional deflection valve is alternately in closed position and inopen position; and generating the level data as a function of the offsetfactor.
 15. The method according to claim 14, further comprising:storing the level data in a memory of the control circuit.
 16. Themethod according to claim 14, wherein the control circuit performs alevel measurement by proceeding successively by: a setting of theadditional deflection valve between the pressure sensor and the upperopening of the pneumatic tube into open position when the deflectionvalve along the pneumatic tube is in closed position; a determination ofthe offset factor of the pressure sensor as a function of a measurementobtained from the pressure sensor; a setting of the additionaldeflection valve between the pressure sensor and the upper opening ofthe pneumatic tube in closed position; a setting into operation of thegas generator for opposing a liquid pressure in the pneumatic tube untilobtaining a balanced pressure at the lower opening of the pneumatictube; a processing of a first measurement obtained from the pressuresensor corresponding to a reading of pressure at the lower opening ofthe pneumatic tube; a setting of the deflection valve along thepneumatic tube in open position; a processing of a second measurementobtained from the pressure sensor corresponding to a reading of pressureat the port of the deflection valve along the pneumatic tube; a settingof the deflection valve along the pneumatic tube back in closedposition. a determination of the calibration coefficient as a functionof a difference between the first and second measurements; and ageneration of the level data based on the pressure measurements as afunction of the calibration coefficient and of the offset factor.